refactor(amu): unify ENABLE_AMU and ENABLE_FEAT_AMUv1

So far we have the ENABLE_AMU build option to include AMU register
handling code for enabling and context switch. There is also an
ENABLE_FEAT_AMUv1 option, solely to protect the HAFGRTR_EL2 system
register handling. The latter needs some alignment with the new feature
scheme, but it conceptually overlaps with the ENABLE_AMU option.

Since there is no real need for two separate options, unify both into a
new ENABLE_FEAT_AMU name in a first step. This is mostly just renaming at
this point, a subsequent patch will make use of the new feature handling
scheme.

Change-Id: I97d8a55bdee2ed1e1509fa9f2b09fd0bdd82736e
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/docs/components/activity-monitors.rst b/docs/components/activity-monitors.rst
index dd45c43..5c1c2c2 100644
--- a/docs/components/activity-monitors.rst
+++ b/docs/components/activity-monitors.rst
@@ -6,9 +6,9 @@
 Unit (|AMU|), an optional non-invasive component for monitoring core events
 through a set of 64-bit counters.
 
-When the ``ENABLE_AMU=1`` build option is provided, Trusted Firmware-A sets up
-the |AMU| prior to its exit from EL3, and will save and restore architected
-|AMU| counters as necessary upon suspend and resume.
+When the ``ENABLE_FEAT_AMU=1`` build option is provided, Trusted Firmware-A
+sets up the |AMU| prior to its exit from EL3, and will save and restore
+architected |AMU| counters as necessary upon suspend and resume.
 
 .. _Activity Monitor Auxiliary Counters: